This invention relates to video signal processing systems, especially systems for manipulating television signals in digital form to produce effects such as changes in magnification, shape, orientation or position of the image or part thereof.
In one form of such a system, as described in U.K. Patent Application No. 8108467, which corresponds to U.S. Pat. No. 4,437,121, incoming video signals, after conversion to digital form, are written in a frame store in input raster format, so that successive pixels of the digitized signals are assigned to successive storage locations in the store. They are then read from the storage locations in a different sequence, and this sequence is pre-determined to produce the desired effect when the signals are reproduced. The operation is illustrated in FIG. 1 of the accompanying drawings in which the small circles represent a patch of storage locations in the frame store in which are stored, during the writing operation, the digital signals representing a group of successive pixels in each of a group of successive lines in a field of the input raster. Four adjacent storage locations are denoted as having addresses x.sub.n, y.sub.n ; x.sub.n+1, y.sub.n ; x.sub.n, y.sub.n+1 and x.sub.n+1, y.sub.n+1 respectively, x and y being the along-line and across-line co-ordinates relative to the input raster. In the same figure the small crosses represent the addresses from which a few successive pixels should be read in line 1 of the output signals. Such addresses are selected according to the effect which is desired, and in general the addresses of the pixels in the output signal raster will not coincide with the storage locations in the frame store. Therefore the pixels used to build up the output signals are synthesized by interpolating among the pixels written in the storage locations adjacent the addresses of the output pixels. For example, the output pixel having the address denoted as x.sub.k, y.sub.k in FIG. 1 would be synthesized by interpolating among the input pixels written at the addresses x.sub.n, y.sub.n ; x.sub.n+1, y.sub.n ; x.sub.n, y.sub.n+1 and x.sub.n+1, yn+1. Each address such as x.sub.k, y.sub.k is derived by transforming the address co-ordinates of the pixel in the output signal raster (say x.sub.k, y.sub.k) into the coordinates related to the input signal raster. Consideration of FIG. 1 will indicate that (assuming the line 1 is representative of the output signal raster) the output signals, when read from the frame store will represent the same image as the input signals, but the image would be compressed and rotated compared with the image represented by the input signals.
A system such as described in the preceding paragraph is a flexible means of achieving production effects. A disadvantage is however encountered occasionally when image compression is effected. Reference to FIG. 1 will show that as the compression factor increases to and beyond 2 (magnification equal to 1/2) the output signals will be unable to resolve detail of short wavelength, of the order to 2 pixel dimensions, in the output raster, and the interpolation used to synthesize the pixels to build up the output signals tends to produce noticeable disturbances in the image.